The term "conversion coating" is a well known term of the art and refers to the replacement of native oxide on the surface of a metal by the controlled chemical formation of a film. Oxides or phosphates are common conversion coatings. Conversion coatings are used on metals such as aluminium, steel, zinc, cadmium or magnesium and their alloys, and provide a key for paint adhesion and/or corrosion protection of the substrate metal. Accordingly, conversion coatings find application in such areas as the aerospace, automotive, architectural, can stock, instrument and building industries.
Known methods for applying conversion coatings to metal surfaces include treatment with chromate or phosphate solutions, or mixtures thereof. However, in recent years it has been recognised that the hexavalent chromium ion, Cr.sup.6+, is a serious environmental and health hazard. Similarly, phosphate ions are a serious risk, particularly when they find their way into natural waterways and cause algal blooms. Consequently, strict restrictions have been placed on the quantity of these species used in a number of industrial processes and limitations have been placed on their release to the environment. This leads to costly effluent processing.
In the search for alternative, less toxic conversion coatings, research has been conducted on conversion coatings based on rare earth compounds. However, there is considerable room for improvement in the adhesion and corrosion protection properties of prior rare earth element based conversion coatings and in the time required to deposit those coatings.
Accordingly, it is an object of the present invention to provide a conversion coating for a metal surface and a process for forming a conversion coating on a metal surface which overcome, or at least alleviate, one or more of the disadvantages or deficiencies of the prior art. It is also an object of the present invention to provide an aqueous, rare earth element containing solution for use in providing a conversion coating on a metal surface.